Vacuum cleaners are designed to separate dirt and dust from an airflow. Commonly, an airflow generator (for example, a motor and fan unit) generates an airflow which draws dirt- and dust-laden air into the vacuum cleaner through a dirty air inlet. The airflow then passes through a form of separating apparatus such as a porous bag or a cyclonic separator (see, for example, EP 0 042 723) to remove dirt and dust from the airflow.
Irrespective of the type of separating apparatus used, there may be a risk of a small amount of dirt and dust passing through the separating apparatus and being carried to the airflow generator. It is undesirable for dirt and dust particles to pass through the fan of an airflow generator because the fan may become damaged or may operate less efficiently. In order to reduce this problem, some vacuum cleaners include a fine filter in an airflow path between the separating apparatus and the airflow generator (see, for example, GB 2 320 419). This filter is commonly known as a “pre-motor filter” and is used to extract any fine dirt and dust particles remaining in the airflow after it has passed through the separating apparatus.
During normal operation of a vacuum cleaner, fine dirt and dust may be deposited on a pre-motor filter and, after a period of time, it could become blocked. Blockages reduce the efficiency at which a vacuum cleaner operates. Therefore, a pre-motor filter will occasionally need to be replaced or cleaned in order to maintain the performance of the vacuum cleaner. In order to allow cleaning or replacement of the pre-motor filter, it is common for such filters to be removable from a vacuum cleaner.
Once the pre-motor filter has been removed, there is a risk that a user will not replace the pre-motor filter, or will replace it incorrectly. If the vacuum cleaner is operated without the pre-motor filter or with it fitted incorrectly, then there is a risk of the dirt and dust which would normally be captured by the pre-motor filter causing damage to, or failure of, the airflow generator as discussed previously.
Various prior art arrangements address this issue by using a form of interlock to prevent operation of the vacuum cleaner if a filter is not correctly located in the vacuum cleaner. For example, U.S. Pat. No. 5,102,435 discloses a vacuum cleaner including a filter interlock which comprises a projection on the filter and a switch on the vacuum cleaner. The projection engages with the switch when the filter is fitted correctly, allowing the vacuum cleaner to operate. However, when the filter is not fitted correctly, the projection does not engage with the switch and the vacuum cleaner will not operate.
The above arrangement requires contact between two mechanical parts which are located in, or close to, an airflow path. Therefore, there may be a risk that these parts will become clogged with dirt and dust over time, or be prone to mechanical failure. This may prevent the interlock from operating correctly.